Pref service: Add initial support for pref registration.

services/preferences/persistent_pref_store_impl.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SERVICES_PREFERENCES_PERSISTENT_PREF_STORE_IMPL_H_
 #define SERVICES_PREFERENCES_PERSISTENT_PREF_STORE_IMPL_H_
 
 #include <memory>
 #include <string>
 #include <unordered_map>
 #include <vector>
 
 #include "base/macros.h"
 #include "services/preferences/public/interfaces/preferences.mojom.h"
 #include "services/preferences/public/interfaces/tracked_preference_validation_delegate.mojom.h"
 
 namespace base {
 class Value;
 }
 
 namespace prefs {
 
 class PersistentPrefStoreImpl : public PrefStore::Observer {
  public:
+  using ObservedPrefs = std::set<std::string>;
+
   // If |initialized()| is false after construction, |on_initialized| will be
   // called when it becomes true.
   PersistentPrefStoreImpl(
       scoped_refptr<PersistentPrefStore> backing_pref_store,
       mojom::TrackedPreferenceValidationDelegatePtr validation_delegate,
       base::OnceClosure on_initialized);
 
   ~PersistentPrefStoreImpl() override;
 
-  mojom::PersistentPrefStoreConnectionPtr CreateConnection();
+  mojom::PersistentPrefStoreConnectionPtr CreateConnection(
+      ObservedPrefs observed_prefs);
 
   bool initialized() { return !initializing_; }
 
  private:
   class Connection;
 
   void SetValue(const std::string& key,
                 std::unique_ptr<base::Value> value,
                 uint32_t flags);
 
@@ skipping 15 matching lines @@
   std::unordered_map<Connection*, std::unique_ptr<Connection>> connections_;
 
   base::OnceClosure on_initialized_;
 
   DISALLOW_COPY_AND_ASSIGN(PersistentPrefStoreImpl);
 };
 
 }  // namespace prefs
 
 #endif  // SERVICES_PREFERENCES_PERSISTENT_PREF_STORE_IMPL_H_